The present invention is directed to the downhole portion of a measurement while drilling instrument that is provided with a hydraulically powered signaling device. This measurement while drilling instrument is operable to provide output signals indicative of at least geometric and geographic orientations of a selected segment of the well borehole. The output signaling device of the particular instrument having this invention is adapted to pulse modulate the mud flowing within the drill string by selectively restricting the mud flow through a lower portion of the instrument as it exits the instrument drill collar. Typically the measurements are desired at the initiation of drilling following the addition of a segment of drilling pipe to the drill string and the instrument is only operated during these periods.
As drilling resumes, the mud pumps at the earth surface are started and the mud pressure within the drill string increases to a nominal operating level. As this occurs the hydraulic circuit powering device of the instrument extracts its operating power from the mud flow through the instrument drill collar and in turn provides the hydraulic power fluid necessary for operating the hydraulic circuits of the instruments mud modulating or signal generating device.
In the prior hydraulic power unit structures for this character of instrument a telescopic piston assembly has been used that combines the formation of a flow restriction necessary for creating the differential driving pressure with the piston structure of the hydraulic reservoir. One structure representative of this is U.S. Pat. No. 4,235,021, issued Nov. 25, 1980. The present invention improves over difficulties of operating the apparatus of this identified patent, namely, for drilling operations in which the mud flow rate is relatively low. In this situation a very significant restriction around the movable plunger apparatus is required in order to provide a sufficient pressure differential for creating the fluid forces necessary to move the plunger assembly downward. This narrow spacing around the plunger assembly causes vibration of the elements involved due to the substantially increased flow rate through the narrow space. The result of this is damaging of the plunger assembly, increased friction and ultimately failure of the plunger assembly. Also, due to the close spacing of the elements involved, the frictional resistance to movement of the plunger assembly within the instrument is substantially increased thereby reducing the operational effectiveness of the hydraulic power unit.